Optimum focusing in an electron beam welding machine

ABSTRACT

Apparatus for determining optimum focusing of an electron beam at the site of a weld wherein the electron beam is directed onto a sample and the intensity of the infra-red radiation emitted at the point of incidence between the beam and the sample is measured by means of a photoelectric cell. A radiation chopper is placed between the photoelectric cell and the samples in such a manner that particles displaced from the workpiece are deflected away from the photoelectric cell. The focus is adjusted in dependence upon the intensity of the infra-red radiation.

1 ALE'P'LdL. n

United States Patent 11 1 1111 3,760,144

Herzberger et al. I 1 Sept. 18, 1973 [54] OPTIMUM FOCUSING IN ANELECTRON 3,299,250 1/1967 Vilkas et al 219/131 BE WELDING MACHINE3,033,974 5/1962 Schleich et al. 219/117 2,746,420 5/1956 Steigerwald 118/8 1 Inventors: Peter Herzberger, Canomca D1 3,049,618 8/1962 Thome250/495 Cuvegho; Learco Di Piazza, Angera, 3,601,577 8/1971 Meyer et al219/121 EB both of Italy [73] Assignee: European Atomic Energy PrimaryExaminer-J. V. Trllhe Community, Luxemburg, Assistant Examiner-Gale R.Peterson Luxembourg Attorney-Stevens, Davis, Miller and Mosher [22]Filed: May 28, 1971 211 Appl. 190.; 148,113 [571 ABSTRACT Apparatus fordetermining optimum focusing of an [30] F i Application p i i Dataelectron beam at the site of a weld wherein the electron June 2 1970Luxembourg 61 048 beam is directed onto a sample and the intensity ofthe n infra-red radiation emitted at the point of incidence be- [52'] CL219/121 EB tween the beam and the sample is measured by means [51] Cl,Bzsk 15/00 of a photoelectric cell. A radiation chopper is placed [58]Field 121 EM between the photoelectric cell and the samples in such 21 13 a manner that particles displaced from the workpiece 9/12 1 1 250/49 5are deflected away from the photoelectric cell. The [56] ReferencesCited focus is adjusted in dependence upon the intensity of UNITEDSTATES PATENTS the infra-red radiation.

2,089,015 8/1937 Bucknam-et a1 219/135 X 2 Claims, 8 Drawing FiguresPATENTED 3.760.144

SHEEI 1 0F 2 10/ F IG .2.

INVENTOR PETER HERZEBERGER ATTORNEY PATTNTEU 81975 3,760,144

SHEET 2 BF 2 FIG.3A 1 T V v FIG.3C *1 T I V Vk F|G.3D

1' T F|G.3E FIGGF PETER HERZEBERGER ATTORNEY OPTIMUM FOCUSING IN ANELECTRON BEAM WELDING MACHINE BACKGROUND OF THE INVENTION This inventionrelates to electron beam welding and particularly to a method ofdetermining optimum focusing of an electron beam.

Various prior art methods for determining the focusing of an electronbeam on a welding point are based on determining the position of thebeam constriction section externally of the target surface and onsubsequent accurate positioning thereof with respect to the targetsurface.

The best known and oldest method comprises frming a molten line on asample of the same composition as the workpiece to be welded, with theselected voltage, current, speed and welding distance. The vapouremitted by the molten metal is ionized in the beam, emitting visibleradiation, which enables the beam to be observed. The current passingthrough the electromagnetic focusing coils is controlled to bring thebeam constriction section on to the target surface.

It is'well known that the beam constriction section of the beam focusedon to the target surfacedoes not correspond to the section obtained whenthe same beam is analysed externally of the target surface, i.e., whenthe electrons move without encountering a target that they can melt.Thus it can be seen that this method is empirical and inaccurate.

Another prior art method comprises recording the variations in thecurrent absorbed in a Faraday cage when the beam is occulted normally toits axis by a disc or by the edge of a bevelled member and when anoscillating beam passes above a diaphragm of a diameter smaller thanthat of the electron beam.

Another prior art method comprises measuring the variation of thecurrent flowing in a tungsten wire which is moved at constant velocityacross the beam normally to the axis thereof. The constriction sectionis obtained when the time taken for the wire to mask or traverse thebeam is at a minimum value. Another method is based on the principlethat a cross-section of the beam intercepted by a target is an X-raysource, an image of which can be obtained on a sensitive plate disposedin a camera; the image of the minimum diameter corresponds to the beamconstriction section.

None of the above methods can be considered entirely satisfactory.Visual control is empirical and inaccurate; it cannot be usedfor metalswhich emit intense bands of light on ionization of their vapours or forany metal when the power of the beam exceeds a critical valuecharacteristic of the particular metal.

The methods based on measuring electrical current are inconvenient inthat they require complex, fragile and expensive equipment, and thedisadvantages become prohibitive in the case for example, of anindustrial machine in which the beam power exceeds about 5 kW. Themethod based on recording the image of the X-ray source has not only theabove disadvantages but also those arising from the intermittent natureof recording, the inaccuracy of the dimensions of the image on the film,and the time required to develop the latter.

Apart from empirical visual adjustment, in all the systems described,the beam constriction section is located without being disturbed by themelting of the target. During welding, the metal ions modify thetrajectory of the electrons and produce a squeezing of the beam near thesurface of the molten metal. The constriction section is moved and itsdiameter is reduced.

These modifications are independent of the operator and their amplitudeand reproducibility are not known, and consequently they introduce. anadditional risk of random error as regards reproductibility of the formand dimensions of a weld.

Another method comprises monitoring the electronic current flowingthrough the workpiece; by studying its variations as a function of theconstriction current it is possible to determine characteristic pointsso that conclusions as to the beam focusing are possible. Thedisadvantage of this method is that the workpiece to be welded must beinsulated from the remainder of the installation, and this is difficultin industrial apparatus when the workpiece has to be movable inside thevacuum enclosure.

Another prior art method is based on the investigation of thetheoretical focusing. If all the parameters of the problem are known,i.e., electron acceleration voltage, workpiece to gun distance, beamintensity, etc.,

then application of electro-optical formulae will enable the value ofthe concentration current to be calculated either by a computer or by anoperator. This latter method has the disadvantage of being purelytheoretical, it is not direct and it is therefore impossible to allowfor the inaccuracy of the parameters of the problem.

SUMMARY According to the invention there is provided a method offocusing the beam of an electron welding machine, comprising the stepsof directing the beam on to a workpiece, measuring the intensity of theinfra-red radiation emitted by the central point of incidence of thebeam on the workpiece,and adjusting the focus in dependence on saidintensity. 7

To obtain optimum focus, the focus is adjusted to maximum intensity ifthere is only one maximum, and the focus is adjusted to the relativeminimum between two maxima in cases in which there are two maxima.

Further according to this invention there is provided in an electronbeam welding machine, a system for indicating the optimum focus of thebeam on a workpiece comprising infra-red radiation detecting meansarranged with its sensitivity axis directed towards the central point ofincidence of the beam on the workpiece, and providing an output signal,amplifying means for receiving the output signal from the radiationdetecting means and providing an amplified output signal, and indicatingmeans for receiving. the output signal from the amplifying means, andfor providing an indication of the amplitude of the radiation receivedby the radia-;

tion detecting means.

DESCRIPTION THE DRAWINGS A specific embodiment will now be described byway of example 'with reference to the accompanying drawings in which:

FIG. 1 is a part schematic section/part diagrammatic view of an electronbeam apparatus; A

FIG. 2 is a perspective view of a grooved cylinder for use in theapparatus of FIG. 1; and

FIGS. 3A to SF show the variation of photocell output as a function ofelectron beam focusing current, for different beam powers.

DETAILED DESCRIPTION According to Kirchhoffs law the radiation energyflux emitted from the welding point is given by equation (1 101 =KST inwhich K is the absorption coefficient; S is the area of the emissionsurface; and T is the absolute temperature.

In FIG. 1 there is shown part of an electron beam welding machinewherein an electron gun 7 is sealed into the wall of a casing 6. Aworkpiece is disposed along the center of easing 6.

When the machine is in operation, the casing is evacuated and anelectron beam is focused onto the workpiece.

In order to set the focus of the electron beam welding machine beforemaking a weld, the focus is first adjusted using a sample having thesame dimensions and being made of the same material, alternatively apcripheral portion of the material to be welded could be used. The termworkpiece is intended to cover both the sample used for the setting ofthe beam focus, and the actual material to be welded.

A tube 2, also sealed into the casing 6, has its axis passing throughthe welding zone of the workpiece. The tube axis makes an angle 0 withthe axis of the electron beam. A silicon photocell 1 is mounted at theouter end of the tube and receives radiation emitted from the weldingzone of the workpiece. The silicon photocell has the peak of itssensitivity curve situated in the infra-red region.

Mounted in front of the photocell are an infra-red filter 3 and aprotective glass 4. The angle 0 is made small to avoid working thephotocell at low levels of radiation.

To assist the operator, an observation window (not shown) is also sealedinto the casing 6.

A radiation chopper is shown in FIG. 2 and comprises a rotatablecylinder 9 having a number of grooves 10 in the cylindrical surface,running parallel to the axis of the cylinder. For convenience, only onegroove is shown in FIG. 2, although in practice any reasonable numbermay be used. The chopper driven by a motor (not shown) is mounted infront of the protective glass 4 such that as the chopper rotates, theaxis of the grooves pass in turn through the tube axis. The cylinder isrotated at such a speed that metal particles emitted from the weldingpoint of the workpiece are prevented from passing completely through thegrooves. This speed of rotation is given by equation where V is thelinear speed at the circumference of the cylinder; v is the speed of themetal particles; I is the length of the cylinder; and d is the width ofthe grooves.

Since it is preferred to have an A.C. amplifying system to avoidproblems of DC. drift, etc., it can be seen that the chopper performstwo functions, (i) preventing any metallization of the photocell, and(ii) chopping the radiation so as to provide a pulsed output from thephotocell.

. The output pulses 1 1 from the photocell are fed to an amplifier 12which has an adjustable feedback resistor 13 so that the gain can bealtered to permit a range of photocell outputs. This range may be fromapproximately 0.4V peak signal to approximately 0.4V peak signal. Theoutput from amplifier 12 is fed to a rectifier 14 then to an integrator15 and finally to a meter 16.

FIGS. 3A to 3F represent oscillograms photographed directly from thescreen of an oscilloscope (not shown) showing the variations in theoutput voltage of the photoelectric cell 1 as a function of the focusingcurrent of the electron beam welding machine for different weldingpowers. For low powers of less than lkW (FIGS. 3A and 3B) the infra-redradiation emission, i.e., the temperature of the surface of the weld,increases with the concentration of the beam and passes through amaximum at maximum concentration. All that is required, therefore, is tonote the focusing current corresponding to this maximum to obtain amaximum power density.

Above l,7OO W (FIG. 3C) there is a flattening of the top of the curvewih the formation of a hollow which is accentuated when the weldingpower increases (approximately 2,500 W), as shown in FIGS. 3D, 3E and3F. Experience has shown that optimum focusing corresponds to the bottomof this hollow; a narrow and deep nail-shaped weld is then obtained. Aweld corresponding to either of the two maxima is wider but shallower.

The explanation of this phenomenon may be as follows: when theconcentration current varies linearly, the focal spot or impact area,decreases, passes through a minimum, and then increases. Initially, thisspot is large and the power density is low, thus the temperature of theworkpiece increases slightly.

As the concentration increases, the beam energy being combined to asmaller area, a crater filled with molten metal forms, and the surfacetemperature rises until a maximum value is reached.

When optimum focusing is obtained, i.e., when the power density reachesits maximum, the electron impact vaporizes the metal to form a craterwhich is covered with a film of molten metal. The beam then passesthrough this film without yielding much energy to it and impinges uponthe base of the crater, where its energy is liberated in heat form. Thepenetration increases and the energy is concentrated farther and fartheraway from the surface. It can be shown that the surface of the metalthen evolves less heat; there is also probably a modification of theabsorption coefficient K and the detected temperature falls. The focalspot then increases again and the temperature passes through anothermaximum and the above phenomenon and explanation apply to this secondmaximum.

The phenomenon, for the case when there are two maxima, normally has anaxis of symmetry passing through the centre of the hollow, which isillustrated by FIGS. 3D, 3E and SF.

Thus by observing the indication on meter 16 the optimum focus of theelectron beam can be obtained; for low electron beam power the optimumfocus is at the position of the maximum, whereas for high electron beampower the optimum focus is at the position of the minimum between thetwo maxima.

We claim:

1. Apparatus for welding a workpiece comprising a. an evacuated casingfor receiving said workpiece;

b. an electron gun mounted in the wall of said casing for directing anelectron beam onto said workpiece,

c. an infra-red radiation detector mounted within said evacuated casingadjacent said electron gun, said detecting means having a sensitivityaxis directed to receive radiation from the region of said parallel tothe sensitivity axis of said infra-red radiation detector,

. means for rotating said radiation chopper about said electron beam isincident are trapped in said groove as said member rotates,

f. amplifying means coupled to said infra-red radiation detector, and

workpiece on which said electron beam is incident, 5 indicating meanscoupled to the output f i d. a radiation chopper comprising a rotatablememplifying means Said indicating means providing an having at least onegroove extending along the indication of the amplitude of the infra-redradiasurface thereof parallel to the axis of said member, tion emittedfrom the region of Said workpiece on Said member being disposed betweenSaid infra-red which said electron beam is incident as a functionradiation detector and said workpiece with its axis 19 of the currentrequired for focussing said electron beam. 2. Apparatus as defined byclaim 1 wherein said radiation chopper is a cylinder having a number ofcircumferentially spaced grooves extending along the cylindrical surfacethereof parallel to the axis of said cylinder.

the axis of said member to interrupt periodically the radiation fallingon said detector, the speed of said rotation being such that metalparticles emitted from the region of said workpiece on which PatentNo.0,1 Dated September 18, 1973 lnventofls) Peter Herzberger, et 511 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Col 3, line 6, change the formula to read 1 KST Col. 4, line 3, change"0. 4V" first occurrence) to -0. 4MV

(SEAL) Attest:

EDWARD MEIETGHERJR. C. MARSHALL DA'NN Attesting Officer Commissioner ofPatents FORM PC3-1050 (10 69) r qscoMM-oc cove-Pp:

1. Apparatus for welding a workpiece comprising a. an evacuated casingfor receiving said workpiece; b. an electron gun mounted in the wall ofsaid casing for directing an electron beam onto said workpiece, c. aninfra-red radiation detector mounted within said evacuated casingadjacent said electron gun, said detecting means having a sensitivityaxis directed to receive radiation from the region of said workpiece onwhich said electron beam is incident, d. a radiation chopper comprisinga rotatable member having at least one groove extending along thesurface thereof parallel to the axis of said member, said member beingdisposed between said infra-red radiation detector and said workpiecewith its axis parallel to the sensitivity axis of said infra-redradiation detector, e. means for rotating said radiation chopper aboutthe axis of said member to interrupt periodically the radiation fallingon said detector, the speed of said rotation being such that metalparticles emitted from the region of said workpiece on which saidelectron beam is incident are trapped in said groove as said memberrotates, f. amplifying means coupled to said infra-red radiationdetector, and g. indicating means coupled to the output of saidamplifying means, said indicating means providing an indication of theamplitude of the infra-red radiation emitted from the region of saidworkpiece on which said electron beam is incident as a function of thecurrent required for focussing said electron beam.
 2. Apparatus asdefined by claim 1 wherein said radiation chopper is a cylinder having anumber of circumferentially spaced grooves extending along thecylindrical surface thereof parallel to the axis of said cylinder.